1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer or the like. In detail, the present invention relates to an image forming apparatus in which a latent image formed on a latent image carrier through electrophotography, electrostatic recording, iongraphy or the like is visualized with liquid developer which is a liquid developing agent. Then, the developed image on the latent image carrier is transferred to a transfer body. Further, in order to use the latent image carrier repeatedly, after the developed image has been transferred, the residual developer is removed from and thus cleans the surface of the latent image carrier.
2. Description of the Related Art
In such a kind of a wet type image forming apparatus, a developing device is used which develops an electrostatic latent image, formed on a latent image carrier such as a photoconductor, using a toner which includes electrically charged particles. Further, in order to clean the latent image carrier, a cleaning liquid is supplied to the latent image carrier at a portion to be cleaned. There are a device which uses the developer which is contained in the developer tank of the developing device as the cleaning liquid, and a device which uses a special cleaning liquid contained in a cleaning liquid tank. In each of the devices, after the cleaning liquid has been supplied to the latent image carrier and used for cleaning, the cleaning liquid including the residual toner is collected in the above-mentioned developer tank or the above-mentioned cleaning liquid tank, and is used repeatedly.
In the device in which the developer is also used as the cleaning liquid, paper powder adhered on the latent image carrier, lumps of the toner adhered to a cleaning blade and so forth gradually mix with the developer. Then, after image forming has been performed tens of thousands of times, the physical properties of the developer change and thus come to be unsuitable for developing. Accordingly, such an old developer, the physical properties of which have changed, is removed from the developing device and is replaced with a new developer. As a method of treating such an old developer, a method of treating developer for electrostatic photography has been proposed (Japanese Patent Publication No.7-36085). In this method, after an oil coagulating agent is added to the old developer and thus, the old developer is coagulated, the coagulated material is compressed so that a developer solvent is collected therefrom, and the residue is treated as industrial waste. In this method, when the oil coagulating agent is added to the old developer, it is necessary to heat the developer to approximately 80.degree. C. so as to coagulate the developer. A method of treating such an old developer, in which, in addition to the oil coagulating agent, a coagulation facilitating agent is added to the old developer so that heating of the old developer is not needed, has been proposed (Japanese Laid-Open Patent Application No.5-313500).
A cleaning device has been proposed (Japanese Laid-Open Patent Application No.62-145273). In this device, after being used in the cleaning process, the special cleaning liquid is collected in the cleaning liquid tank and is used repeatedly. In the device, in order to maintain the cleaning liquid as a clean liquid, means for detecting the concentration of the toner in the cleaning liquid and means for removing and collecting the toner from the cleaning liquid through electrodeposition are provided. Then, the toner removing and collecting means is operated when the concentration of the toner exceeds a predetermined value.
Further, in order to miniaturize the developing device, use of a high concentration, high viscosity developer has been proposed (see Japanese Laid-Open Patent Application Nos.7-209922, 7-152254, 7-239615). In such a method, the concentration of the toner in the developer, which consists of an insulating liquid with the toner dispersed therein, is set to a high level. For example, in Japanese Laid-Open Patent Application No.7-209922, a method for liquid-developing the latent image formed on the latent image carrier with the toner as visualizing particles, which method is provided with a developing process of supplying the liquid developer to the latent image surface of the photoconductor by bringing the liquid developer having a high viscosity of 100 to 10000 mPa.s with the dispersed toner in high concentration in insulating liquid applied to a developing roller into contact with the photoconductor, has been proposed.
Further, in Japanese Laid-Open Patent Application No.7-209922, a liquid-developing method has been also proposed. In the method, prior to the developing process, a prewetting process is performed. In the prewetting process, a prewetting liquid is coated on the latent image carrier. The prewetting liquid has mold release characteristics and is a chemically inert dielectric liquid. In the prewetting process, a prewetting liquid layer separates the layer of the liquid developer supplied to the latent image carrier from the surface of the latent image carrier. Thereby, unnecessary adhesion of the toner on the latent image carrier is prevented, and thus, image degradation due to adhesion of the toner to non-image portions on the photoconductor is prevented.
However, in the treatment of the old developer in the method in which the developer is also used as the cleaning liquid, when a serviceman removes the old developer from the developing device, adds the oil coagulating agent to the old developer so as to coagulate the old developer, and compresses the coagulated material so as to collect the developer solvent therefrom, manually, a long time is required for replacing the developer.
Further, in the method in which the toner is collected through electrodeposition from the special cleaning liquid contained in the cleaning liquid tank so that the cleaning liquid is maintained to be clean, the rate of collection of the toner from the cleaning liquid through the electrodeposition is in proportion to the size of the electrode and the time of the toner collection. Accordingly, in order to increase the rate of collection of the toner, the device is enlarged and/or the time of the toner collection is elongated.
Also, a filter can be used instead of using electrodeposition. The filter is used to trap only the toner and cause the developing solvent to pass therethrough. Thus, the toner is collected from the cleaning liquid. However, similar to the method using electrodeposition, the rate of collection of the toner is in proportion to the size of the filter and the time for the toner collection. Accordingly, in order to increase the rate of collection of the toner, the device is enlarged and/or the time for the toner collection is increased.
Further, when the liquid-developing method provided with the prewetting process is performed in the transfer type electrophotographic apparatus, the developer and also the prewetting liquid remain on the photoconductor after the transfer process. This residual prewetting liquid is then collected in the cleaning process together with the developer. Therefore, when the developer in the developer tank of the developing device is used as the cleaning liquid, and the developer which has been used as the cleaning liquid is returned to the developer tank, the amount of prewetting liquid in the developer tank increases gradually. When the kind of prewetting liquid is different from the kind of developer solvent of the developer, the characteristics of the developer change and thereby the developer may not be used for developing. Specifically, when the liquids of different kinds are mixed together, the electric resistance or the like of the developer changes and the developing characteristics may change. Further, when the liquids of different viscosities are mixed together, the viscosity of the developer changes, and thereby, in the device in which a developing belt or a developing roller is used for conveying the developer and the rate of supply of the developer to the latent image carrier depends on the viscosity of the developer, the rate of supply of the developer to the latent image carrier may change and thereby the developing tone may change.
Further, in such a developing device which uses the liquid developer, generally speaking, a drain (drainage hole) is provided at the bottom plate or a side wall near the bottom of the developing device. The drain is used for the old developer to flow therethrough. The drain is usually closed by a rubber plug or the like when the developing device operates. The drain is opened as a result of the rubber plug or the like being removed when the developer is caused to flow out from the developing device, for example, when the developer is replaced.
However, in an image forming apparatus provided with such a developing device, the developing device may be inclined when the drain is opened and the developer flows out therethrough. In such a case, all of the developer in the developing device may not flow out and some amount of the developer may remain in the developing device. In such a case, when the developing device is detached from the apparatus body or the developing device detached from the apparatus is carried, the residual developer may spill from the developing device through the drain.